Before starting, preheat the oven to 450°F. Wash and dry all produce. Pull stems off mushroom caps, then discard. Brush mushroom caps all over with 2 tsp oil (dbl for 4 ppl). Season with 1/4 tsp garlic salt (dbl for 4 ppl) and pepper. Arrange caps on a parchment-lined baking sheet, top-side up. Bake in the middle of the oven until mushrooms are juicy and fork-tender, 10-12 min.

Meanwhile, halve cucumber lengthwise, then cut into 1/4-inch half-moons.. Cut 1 tomato (2 for 4 ppl) into 1/2-inch rounds .Cut 1 tomato (2 for 4 ppl) into 1/2-inch pieces. Halve buns.

Whisk together vinegar, half the mustard, 1/2 tsp sugar and 2 tbsp oil (dbl both for 4 ppl) in a large bowl. Add spinach, then toss to combine. Season with salt and pepper, to taste.

Stir together mayo and remaining mustard in a small bowl. Set aside.

Once mushrooms are fork-tender, flip caps over, then stuff with mozzarella. Roast in the middle of the oven until cheese melts, 3-4 min. Remove from the oven and set caps aside to cool slightly, 5 min. Meanwhile, arrange buns directly on the top rack of the oven, cut-side up. Toast until buns are golden-brown, 3-4 min. (TIP: Keep an eye on buns so they don't burn!)

Spread mustard mayo over buns. Stack portobello mozza caps, crispy shallots, sliced tomatoes and some dressed spinach on bottom buns. Close with top buns. Add cucumbers and chopped tomatoes to to the bowl with remaining dressed spinach. Toss to combine. Divide burgers and salad between plates.